Measuring health literacy: A systematic review and bibliometric analysis of instruments from 1993 to 2021

Background It has been about 30 years since the first health literacy instrument was developed. This study aimed to review all existing instruments to summarize the current knowledge on the development of existing measurement instruments and their possible translation and validation in other languages different from the original languages. Methods The review was conducted using PubMed, Web of Science, Scopus, and Google Scholar on all published papers on health literacy instrument development and psychometric properties in English biomedical journals from 1993 to the end of 2021. Results The findings were summarized and synthesized on several headings, including general instruments, condition specific health literacy instruments (disease & content), population- specific instruments, and electronic health. Overall, 4848 citations were retrieved. After removing duplicates (n = 2336) and non-related papers (n = 2175), 361 studies (162 papers introducing an instrument and 199 papers reporting translation and psychometric properties of an original instrument) were selected for the final review. The original instruments included 39 general health literacy instruments, 90 condition specific (disease or content) health literacy instruments, 22 population- specific instruments, and 11 electronic health literacy instruments. Almost all papers reported reliability and validity, and the findings indicated that most existing health literacy instruments benefit from some relatively good psychometric properties. Conclusion This review highlighted that there were more than enough instruments for measuring health literacy. In addition, we found that a number of instruments did not report psychometric properties sufficiently. However, evidence suggest that well developed instruments and those reported adequate measures of validation could be helpful if appropriately selected based on objectives of a given study. Perhaps an authorized institution such as World Health Organization should take responsibility and provide a clear guideline for measuring health literacy as appropriate.


Results
The findings were summarized and synthesized on several headings, including general instruments, condition specific health literacy instruments (disease & content), populationspecific instruments, and electronic health. Overall, 4848 citations were retrieved. After removing duplicates (n = 2336) and non-related papers (n = 2175), 361 studies (162 papers introducing an instrument and 199 papers reporting translation and psychometric properties of an original instrument) were selected for the final review. The original instruments included 39 general health literacy instruments, 90 condition specific (disease or content) health literacy instruments, 22 population-specific instruments, and 11 electronic health literacy instruments. Almost all papers reported reliability and validity, and the findings indicated that most existing health literacy instruments benefit from some relatively good psychometric properties.

Conclusion
This review highlighted that there were more than enough instruments for measuring health literacy. In addition, we found that a number of instruments did not report psychometric

Introduction
The term 'health literacy' was first used in 1974 in a paper entitled 'health education as a social policy' [1]. Since then, health literacy appeared more frequently in the biomedical literature and believed that it goes beyond the ability to read, write, and understand the meanings of words and numbers in health care settings [2]. The World Health Organization (WHO) defined health literacy as: 'cognitive and social skills that determine the motivation and ability of individuals to access understand and use the information to promote and maintain optimal health' [3]. Later the WHO regional office for Europe defined health literacy as: 'Health literacy is linked to literacy and entails people's knowledge, motivation and competences to access, understand, appraise and apply health information in order to make judgments and take decisions in every-day life concerning health care, disease prevention and health promotion to maintain or improve quality of life during the life course' [4]. Health literacy is believed to have a vital impact on public health through access to and use of health services [5,6]. Low health literacy is associated with poor health status [6,7], frequent use of health services, longer hospital length of stay [5,6], and high mortality [7,8]. In addition, some studies have linked low health literacy to unhealthy behaviors, such as smoking [4,[9][10][11][12], low physical activity [10][11][12], and low use of preventive services [4,7,10]. Essentially, health literacy plays a role in improving health outcomes both at the individual level (reducing health inequalities) and at the societal level (continuous development of health policies) [13].
Therefore, measuring health literacy is fundamental and needs appropriate measures. Among health literacy instruments, the Rapid Assessment of Adult Literacy in Medicine (REALM) [14], the Test of Functional Health Literacy (TOFHLA) [15], and the Newest Vital Sign (NVS) [16] have a long history of application. These instruments have been criticized for a number of reasons, including evaluation of only a few areas of health literacy, inadequacy for use in interventional studies, or lack of development with a health promotion perspective. In addition, most of these scales were developed and used in clinical settings [17].
In a review of the literature from 1999 to 2013, 51 instruments were identified. Of these, 26 were general health literacy instruments, 15 were condition specific (disease or content), and 10 were health literacy instruments in a specific population [18]. In a review by O'Neil et al. on self-administered health literacy instruments, 35 measures were reported (27 original; 8 derivative instruments) [19]. Nguyen et al., in their study, stated that there are more than 100 health literacy instruments, but only a small number of them have been developed using modern guidelines [20]. In addition, there were further review papers with limited focus covering either general measures or papers that reviewed condition and population-specific health literacy measures. A chronological list of selected review papers is provided in Table 1 [20-38]. However, none of the previous reviews assess instruments comprehensively. Thus, to provide insight into the literature, we performed a bibliometric analysis from the start to the end of 2021 to comprehensively review all existing instruments. We thought this might help synthesize evidence and provide a platform for investigators with similar interests to easily select, apply, or appraise an instrument when needed.

Selection criteria
This study included all original papers reporting psychometric properties of health literacy (and e-health literacy) instruments published in English. Papers only published in journals remained in the study, and books and pamphlets, dissertations, papers presented at conferences, etc., were excluded. All publications were screened using the PRISMA guideline [39].

Quality assessment
The quality of papers was evaluated using the Consensus-based Standards for the selection of the health status Measurement Instrument (COSMIN) checklist. The COSMIN initiative aims to improve the selection of health measurement instruments [40]. For the purpose of this review reporting, six criteria (with at least eight items) were considered sufficient, and for each reported item, a score of 1 was assigned, giving a total score of 8. The criteria were reporting: internal consistency, stability (interclass correlation), face/content validity, structural validity (exploratory and confirmatory factor analyses), criterion validity, hypotheses testing (convergent or divergent validity, discriminant or known groups comparison). Then, the quality of psychometric reporting of each measure was categorized as: poor (< 2), fair (2,3), good (4,5), and excellent (� 6).

Data synthesis
The data for each paper were extracted and summarized. The summary then was tabulated by a topic. The following information was provided: author(s)' name, year of publication, validity, and reliability, and type of instruments, including: 'general health literacy instruments', 'condition (disease or content) specific instruments', instruments that were developed for 'specific populations' [18], and e-Health Literacy instruments.

Descriptive findings
The study flowchart is presented in Fig 1. Overall, 4848 papers were identified. After removing duplicates (n = 2336) and irrelevant documents (n = 2175), 361 papers were included in the final review. Of these, 162 papers introduced an instrument, and 199 papers reported translation and psychometric properties for an original measure. Indeed, the original instruments are briefly described in four categories in the following sections.
(such as HIV, HPV, tuberculosis, cholera, and infectious disease-specific) were the second topic of interest in developing health literacy measures. These instruments have also been wellreviewed and validated in relevant studies in terms of validity and reliability (Table 3).
Among the instruments with special content, the most frequently used were oral/dental health literacy and mental health literacy. The parental and maternal, insurance, occupational,  Table 2. General health literacy instruments (1993-2021).

Construct Internal consistency External/ Relative
Davis et al. [14] 1993  complementary, and alternative medicine, the responsiveness of primary care practices, weight-specific childhood, overweight, social determinants of health, and non-specific neck pain health food, were other specific content measures (Table 4).

Population-specific instruments
A number of health literacy instruments were designed for a specific population-or specific demographic population (n = 22). The grouping was based on age (adolescents, adults/elderly adults, and the elderly) or nationality (Korean, Taiwanese, English, Spanish, American, Switzerland, Australian, German, Chinese, Iranian, and Finnish). A list of instruments and their psychometric properties are shown in Table 5.

e-Health literacy instruments
There were 11 electronic health literacy instruments. Of these, the instrument developed by Norman et al. [189] was used more frequently in various studies. A list of instruments is presented in Table 6.

Other versions that reported for an original instrument
There were a number of instruments that translated and validated in other nations with different demographic backgrounds (n = 199). A list of these instruments is presented in Table 7.

Results for quality assessment
As indicated in the methods section, all papers under review were assessed for quality. The results are shown in Table 8.

Synthesis of findings
Numerous instruments have been developed during the past thirty years for measuring health literacy. This review could provide information on 162 instruments. Of these, there were two well-developed instruments: 1. HLQ, which avoided the use of prevailing theories until the later development process, and great care was taken to fully understand the experiences and lives of people, professionals, and healthcare providers [55].
However, they reported limited psychometric properties. Of the remaining instruments, 15 instruments reported proper psychometric properties needed. In addition, there were a number of instruments that were translated and validated to other languages more frequently. A list of instruments is presented in Table 9.    (Table 7). Among the general health literacy instruments the HLS-EU-Q [56], which examines health literacy in three areas (health care, health prevention, and health promotion), has a potential to be used universally.
Despite a large number of general health literacy assessment instruments and specific topics, currently having a unique and international instrument for measuring health literacy is one of the concerns of public health professionals. This study showed that one of the most widely used instruments at the international level is the European Health Literacy Survey (HLS-EU-Q) [56]. During the development process, the English version of the HLS-EU-Q simultaneously was translated into Bulgarian, Dutch, German, Greek, Polish, Spanish, Irish, Austrian [56] and in Asia into Indonesia, Kazakhstan, Malaysia, Myanmar, Taiwan, and Vietnam [292]. Also, the Taiwanese [293][294][295][296]; Norwegian [297]; Japanese [298]; Vietnamese [299] versions of this instrument have been used in various populations, making it one of the most widely used internationally. Given this instrument's relatively wide range of applications, it may be considered a prelude for producing an international instrument for measuring health literacy.
Many instruments were developed to measure health literacy among specific diseases (chronic non-communicable diseases, especially diabetes, hypertension, and cancer). With the    widespread prevalence of chronic non-communicable, there was a strong desire to develop such instruments. As shown in Table 3, among chronic diseases, diabetes has received more attention than other diseases. Among the instruments that consider a specific content (e.g., maternal, parental, environmental, obesity, and weight gain), oral/dental health literacy and mental health literacy have received special attention. Development and psychometric evaluation of health literacy instruments was observed in different countries. We recognized health literacy instruments in different languages such as Korean, Taiwanese, English, Spanish, American, Australian, German, Switzerland, Finnish, Iranian, Chinese, Japanese, Brazilian, Philippines, and Vietnamese. As shown in Table 5, the countries of Southeast Asia, especially China, have a long history of activity in this field. It has also been shown that the American population and the populations of Southeast Asian countries (Chinese, Taiwanese, and Koreans) address a large number of health literacy assessment instruments.
One of the unique features of this study is the reporting of e-health literacy instruments. There were eleven instruments available for measuring e-health literacy ( Table 6). The existence of many different versions of such instruments (Table 7) demonstrates a growing tendency to measure health literacy related to the increasing use of interment and social media by the general public almost everywhere.
Finally, one should note that the most important question is, do we need so many instruments for measuring health literacy? Although one could not prevent investigators from developing new instruments, it is evident that such haphazard development of instruments is not helpful. It seems that we need a core global general health literacy instrument for use around the globe. Then perhaps it is possible to add a few contents/disease-specific, population-specific, or e-health literacy items to the general instruments according to their use. The experience of the European Organization for Research and Treatment of Cancer-EORTC (the Quality of Life Study Group) might be useful to be adapted (https://qol.eortc.org/quality-of-life-group/).

Limitations
The main criterion in extracting information was the availability of the full-text papers. In cases of no access to the original text, the required information was extracted from their abstracts. Otherwise, such studies were removed from the review. In addition, we only reviewed papers that included the word health literacy in the title. Thus there is a risk of missing papers that did not use health literacy in their titles.

Conclusion
This review highlighted that there were more than enough instruments for measuring health literacy. In addition, we found that a number of instruments did not report psychometric

PLOS ONE
Measuring health literacy properties sufficiently. However, evidence suggest that well developed instruments and those reported adequate measures of validation could be helpful if appropriately selected based on objectives of a given study. Perhaps an authorized institution such as World Health

Content specific health literacy instruments
Cormier et al.

Electronic health literacy instruments
Van der Vaart et al. [193] Duong et al. [199] ✓ -✓ ✓ --✓ -Good https://doi.org/10.1371/journal.pone.0271524.t008 Table 9. A list of instruments that well developed, reported proper psychometric properties, and instruments frequently translated or validated in other countries (1993-2021). Organization should take responsibility and provide a clear guideline for measuring health literacy as appropriate.

Acknowledgments
The authors are grateful to all staff in Iranian Institute for Health Sciences Research, ACECR, Tehran, Iran for help and support.